/Engine/Source/Runtime/Core/Public/Math/UnrealMathUtility.h
Returns closest point on a triangle to a point. The idea is to identify the halfplanes that the point is in relative to each triangle segment "plane"
Point on triangle ABC closest to given point
static FVector ClosestPointOnTriangleToPoint ( const FVector & Point, const FVector & A, const FVector & B, const FVector & C )